This invention pertains to microlithography (projection-exposure) of a pattern, defined on a reticle, onto a substrate using a charged particle beam as an energy beam. Microlithography is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to alignment marks and reference marks, used for high-accuracy positioning of the reticle and substrate, that are irradiated by a charged particle beam, and to microlithography apparatus utilizing such marks.
In charged-particle-beam (CPB) microlithography systems, a reticle (also termed a mask) defines a particular pattern to be projection-transferred to a suitable substrate (e.g., semiconductor wafer). Normally, the reticle is mounted to a reticle stage, and the substrate is mounted to a substrate stage. To ensure accurate pattern transfer, xe2x80x9cmarksxe2x80x9d are situated on the reticle and/or reticle stage as well as on the substrate and/or substrate stage for use as positional as well as other adjustment references.
For example, reference marks usually are provided on the substrate stage for beam-adjustment purposes. Representative beam adjustments include adjustments to image focus, image demagnification, and image rotation. Other uses for such marks include calibration (e.g., measurement of correction values) and baseline checks (in which the position of the optical axis of the microlithography system is checked against the optical axis of the alignment optics).
A schematic elevational section of a conventional reference mark 15xe2x80x2 is shown in FIG. 3. The reference mark 15xe2x80x2 includes a reference-mark substrate 45 made of ZERODUR(copyright) (manufactured by Schott, Germany). A mark pattern is formed on the surface of the mark substrate, of which mark pattern a single mark element 43 is shown. The mark elements 43 are made of a xe2x80x9cheavyxe2x80x9d metal such as Ta or W. The mark elements 43 and remaining portions of the mark substrate 45 are coated with a layer 41 of a xe2x80x9clightxe2x80x9d metal such as Cr. Relatively thick mark elements 43 provide high-contrast of a projected image of the mark 15xe2x80x2. However, because the reference mark 15xe2x80x2 comprises extremely fine (i.e., very narrow) mark elements, the maximum practical thickness of the mark elements is approximately 1 xcexcm. Furthermore, based on various factors (backscattered-electron contrast, thermal expansion of the metal films, ease of manufacturing, etc.), the layer 41 of light metal usually has a thickness of approximately 0.1 xcexcm. (The items denoted with the reference numeral xe2x80x9c47xe2x80x9d are charged particles trapped in the mark substrate 45.)
Unfortunately, the conventional reference-mark structure summarized above can be damaged by a highly accelerated beam of charged particles passing through the light-metal layer 41 into the mark substrate 45. That is, most low-thermal-expansion materials have a high electrical resistance. Whenever an incident highly-accelerated charged particle passes through the light-metal layer 41 and heavy-metal mark feature 43 and penetrates into the low-thermal-expansion material of the mark substrate 45, electrostatic-charging and discharging events occur that damage the mark substrate 45 and surficial features thereon.
It is important that reference marks remain in a stable, usable condition for a long period of time. Hence, surface damage of reference marks is a serious problem.
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide highly durable reference marks suitable for accurate mark-position sensing. Another object is to provide charged-particle-beam (CPB) microlithography apparatus that include such reference marks and that are capable of projection-transferring extremely fine patterns very accurately.
According to a first aspect of the invention, reference marks are provided for use in charged-particle-beam microlithography. An embodiment of such a mark comprises a substrate made of a low-thermal-expansion material. A layer of an electrically conductive material is situated on the surface of the substrate. The layer is connected to electrical ground and has a thickness sufficient to prevent penetration of incident charged particles through the layer to the substrate. At least one reference-mark element is situated on the surface of the layer. The reference-mark element is formed of a material that emits backscattered electrons when irradiated by incident charged particles.
Desirably, the substrate is made of a glass ceramic material such as ZERODUR, described above. Desirably, the conductive layer is made of a light metal having a thickness ranging from 2 to 20 micrometers. Further desirably, the material that emits backscattered electrons is a heavy metal.
In an especially advantageous configuration, multiple reference-mark elements are arranged into a line-and-space pattern.
In a CPB microlithography apparatus according to the invention, the reference mark can be located on a substrate stage or on a microlithographic substrate mounted on the substrate stage.
The invention also encompasses semiconductor-fabrication processes utilizing CPB microlithography apparatus according to the invention, and semiconductor devices made according to the processes.
The foregoing and additional advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.